Angular velocity sensors, such as gyroscopes ("gyros"), are sometimes used in vehicles to measure changes in heading. One particular application of a gyro is to measure changes in heading in an on-board navigation system for an automobile, which is designed to assist the driver of the automobile in navigating. Such systems are available from Zexel Innovation, Inc., of Sunnyvale, Calif.
Calibration is a problem that is associated with gyros. In particular, a gyro normally outputs a voltage or other equivalent signal which represents the current angular velocity detected by the sensor. To determine the true angular velocity, the output voltage of the gyro is compared with the output voltage level that corresponds to an angular velocity of zero-the difference indicates the true angular velocity. Thus, the output voltage level that corresponds to an angular velocity of zero is essentially a reference level, which may be referred to as the "gyro zero voltage". Accurate determination of angular velocity, therefore, depends upon accurate knowledge of the gyro zero voltage. However, the output of the gyro has a tendency to drift in response to changes in temperature and humidity and other factors. Consequently, navigation systems which employ these sensors typically employ some technique to determine and update the gyro zero voltage.
One approach to determining the gyro zero voltage is to measure the output of the gyro each time the vehicle stops and to set the gyro zero voltage equal to the measured value. It may be difficult, however, to reliably determine when the vehicle is stationary. In an automobile navigation system such as mentioned above, one approach to determining when the vehicle is stationary is to observe the rotation of the vehicle's wheels. Wheel rotation may be detected, for example, from the pulses that are typically used to increment the vehicle's odometer (distance sensor). The vehicle may be considered to be stationary if no pulses are observed for a predetermined time interval. However, if the vehicle is moving very slowly, it is possible that no pulses will be observed during the time interval even though the vehicle is moving. Hence, the vehicle may actually have a non-zero angular velocity, such that the output of the angular velocity sensor may not accurately represent the gyro zero voltage.
Hence, what is needed is an improved technique for determining a gyro zero voltage. In particular, what is needed is an improved technique for determining when a vehicle in which a gyro is installed is stationary.